Conventional concrete lined steel water pipe, e.g., large diameter pipe, is currently designed to a maximum working stress in the steel of around 21,000 psi. The reason for limiting the allowable design stress in such pipe is the limited allowable strain in the concrete or mortar lining. A higher steel stress will crack the concrete or mortar lining under pressure, causing deleterious cracks to form, adversely affecting the lining's performance, and creating the potential for the lining to fail or otherwise fall out of the inside of the steel pipe.
The use of concrete or mortar linings with such steel pipe is well known for use in the water pipe industry because they have a proven record of protecting the steel pipe from interior corrosion for a long period of time, e.g., of more than 50 years. As an alternative to concrete linings, polymer coatings such as epoxy based coatings and the like can be used with steel pipes that can be designed having a higher strain to failure. However, no currently available polymer coating can be guaranteed or expected to last 50 years in water service without some form of periodic maintenance, typically at approximately 15 year intervals. It is not practical to take such coated steel water pipes, e.g., when used as a water main or the like, out of service to sandblast and recoat the lining every 15 or so years for maintenance purposes.
A second limitation for concrete lined steel water pipe is the engineering consideration and need to have a diameter to thickness ratio of around 240, and preferably less than about 220. This means that a 100-inch diameter pipe must have a minimum pipe wall thickness of approximately 0.417-inch (D/t=240; t=100/240=0.417). The effect of this design requirement is that pipe which could have been designed with a lesser wall thickness due to internal pressure, requires a greater thickness due to handling, shipping and installing stresses. In this 100″ ID×0.417″ wall example, pipe rated at P=(2*t*stress)/D=2*0.417*21,000/100=175 psi or less must still have a pipe wall thickness based on D/t=240. The D/t requirement is based on the practicality of handling, shipping and installing the pipe. In poor soil conditions, the pipe is also vulnerable to collapse or over deflection if not stiffened by increased liner thickness, steel pipe wall thickness, attached stiffeners, or the expensive importation of more stable bedding materials.
Conventional prestressed concrete cylinder pipe (PCCP) comprises an inner concrete pipe or core that is placed in state of high compression by the use of wires that are wound around the inner pipe. The pressure loads that are placed on the pipe in service are taken by the prestressed steel wires using high allowable design stress, therefore also operating to reduce the amount of steel that is used in providing the desired amount of compression. The concrete core resists the prestressing wires and helps support the earth loads in a buried condition. The prestressed concrete core also helps to support the pipe during transport and installation.
One of the problems, however, with PCCP is the difficulty in protecting the high strength prestressing wires from corrosive environments when the pipe is placed into service, and the potential susceptibility of such wires to hydrogen embrittlement if excessive levels of cathodic protection are applied. The ability to easily cathodically protect the prestressing wire is further complicated by the low dielectric strength of the typical mortar coating that is placed over the prestressing wires to protect them from corrosion. The dielectric resistance can be improved by the application of a suitable polymer coating, e.g., such as one available from Ameron International under the product name Amercoat 1972B. While the use of such a polymer coating makes it easier to cathodically protect the steel prestressing wires, it does so at an additional cost in terms of both manufacturing steps and in terms of raw material cost.
As the underground utility complex of pipes and surface or subsurface transit systems in typical major cities have increased over the years, another problem has arisen from “cathodic interference” or stray ground currents caused by nearby steel pipelines under cathodic protection for exterior corrosion, and caused by DC powdered transit systems. Large water pipes installed many years ago now find new pipes that have been installed nearby. These new pipes may be cathodically protected, and the stray ground currents introduced from the cathodic protection can cause corrosive currents in the PCCP. As a result of the possible existence of such stray ground currents, many metropolitan water agencies now prefer either conventional steel pipe with a dielectric coating for external corrosion protection, or steel cylinder reinforced concrete pipe when external loads are high. Although this is an effective approach for controlling pipe corrosion, it is costly.
One approach known in the art for making prestressed steel pipe has been to use a multi-step process of first forming/casting a concrete core, slipping a steel pipe over the pre-cast concrete core, and then pumping a high-pressure grout into the annular gap between the pre-cast concrete core and steel pipe to place the concrete core under compression. However, two major problems exist with this method of making pipe. A first problem relates to the difficulty in perfectly sealing the ends of the concrete core to the steel pipe during the pressure grouting operation. A second problem involves how to apply and maintain pressure to the grout during the cure of the grout, especially if minor leakage of the grout occurs at the seals between the steel pipe and concrete core while the grout is curing. If a high-pressure grout pump is used, the potential leakage of grout during cure must be made up by the grout pump. This will mean the grout pump will have to be active during the curing operation in order to make up for leakage, which leaves the potential for the grout to cure in the pump, both ruining the pump and resulting in pipe having an insufficient level of prestress.
It is, therefore, desired that a pipe construction and method for making the same be developed that is capable of providing a desired level of properties such as pipe stiffness and internal corrosion resistance that matches the typical minimum 50 year service life offered by conventional PCCP or concrete lined steel pipe. It is further desired that such pipe construction be capable of providing a desired degree of protection from external corrosion and cathodic interference. It is still further desired that such pipe construction be manufactured in a manner that is both cost efficient from a raw material perspective, and from the amount of time and labor involved in making the same.